Stimulated echo magnetic resonance (MR) imaging methods were first described several decades ago. The basic concept is to tip the proton magnetization into the longitudinal (-z) direction, wait for a period of time, and then tip the magnetization back into the transverse (x-y) plane. This results in the production of a stimulated-echo magnetic resonance signal. However, such methods are intrinsically slow, because of the need to allow time for longitudinal (T1) relaxation between sequential acquisitions of the multiple phase-encoding steps needed to create an MR image. Images with low spatial resolution have been acquired by using a process consisting of radio frequency pulses, a dephasing magnetic field gradient, a single shot gradient-echo type acquisition, and a dephasing gradient placed after each radio frequency pulse. However, this method is not generally useful for clinical applications because of artifacts and because the spatial resolution is not sufficient.
Diffusion is a process of random translational motion of molecules. Methods for using nuclear magnetic resonance to study diffusional processes have been proposed including a procedure in which the signal intensity is related to these diffusional processes. Using animal models of brain ischemia, for example, it has been shown that the measurement of diffusion was a sensitive indicator of ischemia.
A continuing need exists, however, for further advances in systems and procedures used in performing magnetic resonance imaging for diagnostic purposes and in particular for measuring and imaging of diffusion in biological tissues.